The present invention relates to a separator for non-aqueous electrolyte batteries which provides superior processability in fabrication of batteries, does not shrink or burn even when heat is generated due to external short-circuit of electrodes, causes no internal short-circuit owing to contact between electrodes, can inhibit ignition of batteries, and can provide high energy density and excellent cycle life; a non-aqueous electrolyte battery using the same; and a method for manufacturing the separator for non-aqueous electrolyte batteries.
Hitherto, porous materials comprising polyolefins such as polypropylene and polyethylene have been used as separators for non-aqueous electrolyte batteries such as lithium secondary batteries. For example, JP-A-6-325747 discloses a microporous film comprising a high-molecular weight polyethylene having an intrinsic viscosity (xcex7) of 5 dl/g or higher as a separator for non-aqueous electrolyte batteries. JP-A-6-163023 discloses a microporous film comprising a mixture of polyethylene and ethylene-propylene rubber as separators of lithium primary and secondary batteries.
These separators have shutdown function for the prevention of ignition of batteries. The shutdown function is a function to prevent melting and ignition of Li upon the battery temperature reaching 180xc2x0 C. in case the electrodes are short-circuited and a great electric current flows to generate heat. Specifically, before occurrence of the ignition of Li, separators are molten and openings thereof are clogged, whereby the battery reaction is stopped to inhibit generation of heat.
For example, the batteries are designed so that when a porous material of polyethylene is used as separators, the shutdown takes place at about 120xc2x0 C., and when a porous material of polypropylene is used as separators, the shutdown takes place at about 140xc2x0 C., whereby generation of heat in the battery is stopped to inhibit increase of temperature. However, in the case of generation of excess heat which cannot be suppressed by the shutdown function, melting of the separators proceeds to cause cracking due to complete melting or fusion of the separators, resulting in contact between electrodes, and, as a result, the short-circuit current again flows to cause generation of heat and ignition of battery.
Furthermore, since these separators are low in tear strength and penetration strength, they may be ruptured or broken through by projections of electrodes or by accidents at the time of fabrication of batteries.
JP-A-5-151949 discloses multi-layer separators for batteries which comprise a laminate of a polyolefin microporous thin film and a polyolefin nonwoven fabric, the laminate being hot pressed at a temperature lower than the melting points of the materials constituting the thin film and the nonwoven fabric. However, since the materials of the laminate are all polyolefins, they are inferior in heat resistance and cannot prevent internal short-circuit in case the temperature rises to such an extent as cannot be suppressed by the shutdown function.
For the improvement of heat resistance of separators, it is effective to add inorganic materials such as glass, alumina and ceramics, or resins and fibers superior in heat resistance. However, these materials generally contain polar groups such as hydroxyl group, silanol group and carboxyl group which adversely affect battery characteristics and, thus, these materials cannot be used as they are.
For increasing the strength of separators, the following methods are effective: a method of mixing with pulp to utilize the interlocking of pulp; a method of bonding with polyvinyl alcohol, ethylene-vinyl alcohol copolymer or the like to increase the strength; and a method of making composites with woven fabric, nonwoven fabric, paper or the like. However, pulp, polyvinyl alcohol and ethylene-vinyl alcohol copolymer contain hydroxyl group which adversely affects battery characteristics, and if woven fabric, nonwoven fabric and paper comprises materials containing polar groups such as hydroxyl group, silanol group and carboxyl group which adversely affect battery characteristics, when these are used as separators for non-aqueous electrolyte batteries, the battery characteristics such as energy density and cycle life are considerably deteriorated.
For example, JP-A-7-220710 discloses a separator for batteries, characterized by comprising a paper mainly composed of cellulose fibers and a polyethylene microporous film having micropores, for the purpose of providing a separator for batteries which blockades between positive electrode and negative electrode before the rise of temperature inside the battery reaches a dangerous zone and, besides, is diminished in the danger of rupture when the temperature further rises, and keeps insulation between positive electrode and negative electrode.
For providing a separator for batteries having good shutdown properties and heat resistance, JP-A-9-213296 discloses a battery separator, characterized by a sheet of laminate structure comprising a heat-unfusible microporous layer of a sheet made of a mixture of cellulose fibers and heat-unfusible synthetic fiber fibrils finely divided to a water retentivity of 210-450% and a heat-fusible micro-porous layer comprising a polyolefin resin, these layers being superposed.
However, the separators of JP-A-7-220710 and JP-A-9-213296 suffer from the problem that since hydroxyl group contained in cellulose fibers adversely affects the battery characteristics, energy density and cycle life are considerably deteriorated when used as separators for non-aqueous electrolyte batteries.
For providing a separator which prevents internal short-circuit caused by the contact between positive and negative electrodes, JP-A-7-302584 discloses a separator for batteries, characterized by comprising a nonwoven fabric containing at least 50% by weight of micro-fibrillated fibers of an organic synthetic polymer which have an average fiber length of 0.2-1.5 mm and an average fiber diameter of 0.05-1 xcexcm.
However, since the microfibrillated fibers comprise an organic synthetic polymer, binding force of the fibers per se is small, and when a porous base is made using at least 50% by weight, especially 100% of the fibers, the fibers fall off from the porous base or the base is considerably low in tear strength and penetration strength to cause problem in rollability together with electrodes.
JP-A-2-170346 discloses an inorganic non-aqueous electrolyte battery having a negative electrode of an alkali metal, a positive electrode comprising a porous molded body mainly composed of carbon, a separator interposed between the negative electrode and the positive electrode and an electrolyte containing a solvent of an oxyhalide which is a positive electrode active material, wherein said separator comprises a glass fiber nonwoven fabric made using a binder mainly composed of polyethyl acrylate or a copolymer of ethyl acrylate and acrylonitrile and containing an organosilane compound.
In this case, the organosilane compound is used in order to improve binding force between the binder and the glass fibers and further increase tensile strength of the glass fiber nonwoven fabric by adding to the binder which is used for increasing tensile strength of the glass fiber nonwoven fabric.
The glass fiber nonwoven fabric here is a nonwoven fabric mainly composed of glass fibers. Therefore, when the tensile strength is increased with binder, rigidity is also increased, and, hence, the nonwoven fabric is readily broken and rollability together with electrodes is inferior, resulting in inferior battery fabricability.
Furthermore, the effect of the organosilane compound is that a part of the organosilane compound dissolves into the electrolyte and prevents densification of the alkali metal halide film which is produced on the surface of negative electrode by the reaction of oxyhalide as a positive electrode active material with alkali metal of the negative electrode and, as a result, reduction of voltage at the initial discharge after storage at high temperatures or for a long time can be inhibited.
Accordingly, the above technique is clearly different from the technique of improving heat resistance of the separator by applying an organometallic compound to a porous base constituting the separator for non-aqueous electrolyte batteries or by forming a film and of improving battery characteristics such as energy density and cycle life by blocking the polar group which adversely affects the battery characteristics.
For the purpose of providing a secondary battery which can be prevented from short-circuit of negative electrode and positive electrode inside the battery even if dendrite is formed at the negative electrode, JP-A-6-196199 discloses a secondary battery comprising a negative electrode composed of a negative electrode active material and a positive electrode composed of a positive electrode active material which are separated by a separator, wherein at least a multi-layer metal oxide is provided between the positive electrode and the negative electrode.
In the above battery, the short-circuit of the negative electrode and the positive electrode in the battery can be inhibited even if dendrite is formed at the negative electrode by using, as a part of separator, a multi-layer metal oxide film formed by molding a bimolecular film forming compound in a mold, and the multi-layer metal oxide film per se serves as a separator. However, the multi-layer metal oxide film is very low in strength and it cannot be used alone as a separator. Therefore, the multi-layer metal oxide film must be formed as a part of a general separator or a porous base used as a support or it must be interposed between other separators. For this reason, there are problems that the separators or porous bases usable as supports are limited to the poly-olefin resins or fluorocarbon resins and therefore they are low in tear strength and penetration strength, and the separators are crushed or broken at the time of fabrication of batteries, and besides are inferior in heat resistance or cannot provide sufficient energy density.
Moreover, the multi-layer metal oxide film per se is a separator, and this is clearly different from the technique of directly reacting an organometallic compound with a polar group such as hydroxyl group, silanol group or carboxyl group contained in the porous base constituting a separator for non-aqueous electrolyte batteries to bond the organometallic compound to the porous base or to form a film, thereby to improve heat resistance of the separator and improve battery characteristics such as energy density and cycle life.
JP-A-6-168739 discloses a secondary battery comprising at least a negative electrode, a separator, a positive electrode, an electrolyte, a collector and a battery case, wherein at least the surface of the positive electrode that faces the negative electrode is covered with one or two or more layers of films selected from an insulator, a semiconductor and a composite of an insulator and a semiconductor which are permeable to ions which participate in the battery reaction.
According to this technique, even if dendrite of lithium or zinc grows at the time of charging, short-circuiting of negative electrode and positive electrode is inhibited by forming on the surface of positive electrode a film of an insulator, a semiconductor or a composite of an insulator and a semiconductor permeable to ions which participate in battery reaction. This is clearly different from the technique of applying an organometallic compound to a porous base constituting a separator for non-aqueous electrolyte batteries or forming a film of the organometallic compound to improve heat resistance of the separator and block the polar group which adversely affects the battery characteristics, thereby to improve battery characteristics such as energy density and cycle life.
For the purpose of providing a separator which is excellent in chemical resistance, heat resistance and hydrophilicity, causes no uneven wetting and can attain improvement of characteristics of apparatuses for electrochemical reaction, JP-A-8-250101 discloses a separator for apparatuses for electrochemical reaction, characterized by comprising a porous body of a composite of a metal oxide and a polymer, said porous body being produced by covering at least fine fibers, fine knots or wall surface of pores of a polymer porous body having interconnecting pores with a metal oxide comprising a dry body of a metal oxide hydrous gel formed by gelling reaction of hydrolyzable metal-containing organic compound.
This separator suffers from the problem that since the metal oxide contains a slight amount of water, energy density or cycle life is deteriorated due to the water contained in the metal oxide when it is used as a separator for non-aqueous electrolyte batteries.
JP-A-3-124865 discloses a heat resistant fiber nonwoven fabric made by bonding the fibers with a hydrolyzed condensate comprising an acid, water and one or two or more compounds selected from methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane and ethyltriethoxysilane. Moreover, as a different use, JP-A-5-64712 discloses a glass fiber filter paper for heat resistant air filters which contains a condensate produced from an alkoxysilane in an amount of 1-7% by weight based on the total weight of the filter paper. As a different use, JP-A-7-328355 discloses a dust-removing filter made of glass fibers the surface of which is treated with an organosilane having a hydrolyzable group.
These heat resistant fiber nonwoven fabric, glass fiber filter paper for heat resistant air filter and dust-removing filter made of glass fibers have problems in fabricability of batteries when they are used as separators of non-aqueous electrolyte batteries. That is, since they are poor in interlaminar strength and folding strength, when the separator is rolled together with electrodes, they are readily crashed through or broken by the projection of the electrodes or by accident and, besides, they are apt to generate dust due to friction or shock and are readily broken once a fold is given. Even if they can be rolled, interlaminar separation is apt to occur, thickness becomes uneven, and they lack flexibility. Thus, the separators are inferior in adhesion to electrodes and may slip off from the electrodes to produce a space, resulting in non-uniform electric resistance inside the battery. Therefore, these separators, as they are, cannot be used as separators of non-aqueous electrolyte batteries.
The inventors filed WO/96/30954 on nonwoven fabrics for separators of non-aqueous electrolyte batteries and non-aqueous electrolyte batteries using the same. The nonwoven fabrics for separators of non-aqueous electrolyte batteries disclosed in the above patent application has the following features: the thickness uneven index Rpy in machine direction is 1000 mV or less; they have good adhesion to electrodes and give superior battery fabricability such as rollability with electrodes; even if heat is generated due to external short-circuit of electrodes, no internal short-circuit occurs which is caused by the contact between electrodes brought about by shrinking or burning of the nonwoven fabrics, and thus ignition of battery can be prevented; and battery shelf stability is excellent.
However, even said nonwoven fabrics for separators of non-aqueous electrolyte batteries cannot still provide satisfactory energy density and cycle life.
The present invention solves the above problems in the prior art. That is, the object of the present invention is to provide a separator for non-aqueous electrolyte batteries which neither breaks nor slips off to produce no spaces between electrodes at the time of fabrication of battery, is good in rollability with electrodes and results in excellent battery fabricability, does not shrink or burn even if external short-circuit of electrodes occurs, causes no internal short-circuit which is caused by the contact between electrodes, to prevent ignition of battery, and gives a high energy density and an excellent cycle life. Further objects are to provide a non-aqueous electrolyte battery using the separator and a method for manufacturing the separator.
The present inventor have conducted intensive research in an attempt to solve the above problems and accomplished the present invention.
That is, the present invention relates to a separator for non-aqueous electrolyte batteries which comprises a porous base comprises at least one material selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper and an organometallic compound applied to the porous base.
It is preferred that the porous film has a maximum pore diameter of 10 xcexcm or less measured by bubble point method specified in ASTM F-316-80.
It is preferred that the porous base has a maximum pore diameter of 20 xcexcm or less measured by bubble point method specified in ASTM F-316-80.
It is preferred that the nonwoven fabric or paper has a maximum pore diameter of 20 xcexcm or less measured by bubble point method specified in ASTM F-316-80.
It is preferred that the porous base contains inorganic fibers.
It is preferred that the inorganic fibers are micro-glass fibers comprising a silica glass containing 99% by weight or more of silicon dioxide (in terms of SiO2) or an E glass containing 1% by weight or less of sodium oxide (in terms of Na2O).
It is preferred that at least one of organic fibers are heat resistant organic fibers having a melting point or a heat decomposition temperature of 250xc2x0 C. or higher.
It is preferred that at least a part of the organic fiber are fibrillated to 1 xcexcm or less in fiber diameter.
It is preferred that the organic fibers at least a part of which are fibrillated to 1 xcexcm or less in fiber diameter are at least one kind of fibers selected from vegetable fibers, vegetable fiber pulps, bacteria celluloses produced by micro-organisms, rayons, polyolefin fibers, polyamide fibers, aramid fibers and polyarylate fibers.
It is preferred that the porous base contains polyvinyl alcohol.
It is preferred that the porous base is subjected to pressing treatment or hot pressing treatment.
It is preferred that the organometallic compound is at least one compound selected from organosilicon compound, organotitanium compound, organoaluminum compound, organozirconium compound and organozirco-aluminate compound.
It is preferred that the organometallic compound is an organosilicon compound.
It is preferred that the organosilicon compound is at least one compound selected from organosilanes or organopolysiloxanes having at least one hydrolyzable group or functional group selected from chlorine group, fluorine group, acetoxy group, alkoxy group, vinyl group, amino group, epoxy group, mercapto group and methacryl group.
The method for producing a separator for non-aqueous electrolyte batteries according to the present invention is characterized in that a porous base comprising at least one member selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper is allowed to contact with a solution of an organometallic compound by a method of impregnation, coating or spraying and is dried or cured by heating to apply the organometallic compound to the porous base.
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, at least one porous base (A) selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper or (A) and a porous base (B) containing no organic fibers are previously allowed to contact with a solution of an organometallic compound by a method of impregnation, coating or spraying and dried or cured by heating to apply the organometallic compound to the porous base to make a composite (C) comprising a combination of (A) or a combination of (A) and (B).
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, a composite (C) comprising a combination of at least one porous base (A) selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper or a combination of (A) and a porous base (B) containing no organic fibers is made and then this composite (C) is allowed to contact with a solution of an organometallic compound by a method of impregnation, coating or spraying and dried or cured by heating to apply the organometallic compound to the composite.
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, a wet sheet obtained by wet paper making process or the sheet after dried is allowed to contact with a solution of an organometallic compound by a method of impregnation, coating or spraying and dried or cured by heating to apply the organometallic compound to the sheet.
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, a fiber slurry containing an organometallic compound is beaten or macerated and then the fiber slurry alone or a mixed slurry comprising said fiber slurry and other fiber slurry is subjected to wet paper making process and is dried or cured by heating to apply the organometallic compound to the resulting sheet.
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, a fiber slurry containing an organometallic compound is beaten or macerated and then the fiber slurry alone or a mixed slurry comprising said fiber slurry and other fiber slurry is subjected to wet paper making and the resulting wet sheet or the sheet after dried is allowed to contact with a solution of an organometallic compound by impregnation, coating or spraying, followed by drying or curing with heating to apply the organometallic compound to the resulting sheet.
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, the porous base is subjected to pressing treatment or hot pressing treatment.
In the method for producing a separator for non-aqueous electrolyte batteries according to the present invention, pressing treatment or hot pressing treatment is carried out to adjust the maximum pore diameter to 20 xcexcm or less measured by bubble point method specified in ASTM F-316-80.
The separator for non-aqueous electrolyte batteries of the present invention will be explained in detail.
The porous base used in the present invention comprises at least one material selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper. That is, the base comprises one of these materials, a composite comprising a combination of these materials, or a composite comprising at least one of these materials and a porous base containing no organic fibers.
Materials constituting the porous film used in the present invention include, for example, polyolefin resins and fluorocarbon resins, but they are not limited.
The porous film used in the present invention preferably has a maximum pore diameter of 10 xcexcm or less as measured by the bubble point method specified in ASTM F-316-80. Since the pores of the porous film pierce through the film linearly and in the z direction at the shortest direction, if the maximum pore diameter is more than 10 xcexcm, dendrites produced by the repetition of charging and discharging or electrode active materials falling off due to some shock readily pass through the pores of the separator to cause internal short-circuit. Therefore, the smaller pore diameter is the better as far as ion can permeate through the separator, but if the pore diameter is smaller than needed, the electrolyte permeation properties or retention properties are considerably deteriorated to result in extreme deterioration of battery characteristics. Thus, the pore diameter is preferably 0.001 xcexcm or more.
Being different from the pores of the porous film, those of woven fabric, nonwoven fabric and paper used in the present invention are not linear and are intricate. Especially, the pores of the nonwoven fabric and paper comprise complicated passages due to the effect of disordered orientation or interlocking of fibers, and, hence, the dendrite or the electrode active material falling off is more difficult to be passed therethrough than in the case of the porous film.
As the organic fibers contained in the woven fabric or nonwoven fabric used in the present invention, mention may be made of single fibers, composite fiber and various heat-fusible fibers comprising resins such as wood pulps, non-wood pulps, rayons, celluloses, cuprammonium rayons, polynosics, acetates, acrylics, polyolefins, polyesters, polyamides, polyimides, polyamide-imides, polyether ketones, polyether sulfones, polyvinyl alcohols and ethylene-vinyl alcohol copolymers.
The materials constituting the nonwoven fabric used in the present invention, mention may be made of, in addition to the organic fibers, inorganic fibers, inorganic additives and resins superior in heat resistance, such as glass fibers, micro-glass fibers, alumina fibers, alumina-silica fibers, ceramics fibers, zirconia fibers, rock wool, Tyrano fiber, silicon carbide fibers, potassium titanate fibers, alumina whiskers, aluminum borate whiskers, colloidal alumina, colloidal silica, epoxy resins and fluorocarbon resins.
Papers used in the present invention include, for example, so-called papers mainly composed of wood pulp, and non-wood fibers or non-wood pulps such as straw, bagasse, paper mulberry, mitsumata (Edgeworthia papyrifera), Manila hemp, esparto, cotton linter, ganpi (Wikstroemia sikokiana Fr. Et Sav.), jute, bamboo, ditch reed, papyrus, kenaf, and ramie, and filter papers.
As far as the effect to block the polar groups is not reduced so much, the nonwoven fabrics and papers used in the present invention may contain various adhesives such as thermoplastic resins, e.g., vinyl acetate resins, vinyl chloride resins, polyvinyl alcohol resins, polyvinyl acetal resins, acrylic resins, polyamide resins and ethylene-vinyl acetate copolymers, thermosetting resins, e.g., urea resins, melamine resins, phenolic resins, epoxy resins, polyurethane resins, polyester resins, polyaromatic resins and resorcinol resins, and elastomers such as chloroprene elastomers, nitrile rubber elastomers, butyl rubber elastomers, polypropylene elastomers and silicone rubber elastomers.
The porous base used in the present invention preferably comprises a nonwoven fabric or a paper because the pores are formed by disordered orientation and complicated interlocking of fibers, and, hence, the base is excellent in retention of electrolyte; dendrite or electrode active material falling off is difficult to pass through the base; the base is excellent in heat resistance; occurrence of internal short-circuit can be inhibited; the base is high in tear strength and penetration strength; and separators for non-aqueous electrolyte batteries excellent in rollability with electrodes can be obtained.
It is preferred that the porous base in the present invention comprises particularly a nonwoven fabric, because it swells a little after immersed in the electrolyte and is excellent in dimensional stability and, therefore, area of electrodes to be inserted in the battery can be gained and thus non-aqueous electrolyte batteries of high capacity can be produced.
When the porous base in the present invention has a maximum pore diameter of 20 xcexcm or less as measured by the bubble point method specified in ASTM F-316-80, it is excellent in retainability of electrolyte and charging and discharging can be stably repeated. As a result, excellent cycle life and shelf stability of battery can be obtained.
Pore diameter of the nonwoven fabric and paper used in the present invention may be less than that which can be visually noticed and has no special limitation, but the maximum pore diameter is preferably 20 xcexcm or less, more preferably 10 xcexcm or less measured by the bubble point method specified in ASTM F-316-80.
When the maximum pore diameter is 20 xcexcm or less, especially 10 xcexcm or less, since the nonwoven fabric or paper is excellent in retainability of electrolyte and charging and discharging can be stably repeated, excellent cycle life and shelf stability of battery can be obtained.
As the porous base containing no organic fibers, mention may be made of a nonwoven fabric and a mat composed of inorganic fibers, inorganic powders, resins or the like.
The composite comprising a combination of a porous film, a woven fabric or nonwoven fabric containing organic fibers, and a paper and the composite comprising a combination of at least one of them and a porous base containing no organic fibers include a multi-layer material made without bonding the layers and a multi-layer material made by partially or wholly bonding the layers, and these can be combined depending on the purpose.
When the porous base in the present invention comprises the above composite, a multi-functional separator for non-aqueous electrolyte batteries is obtained.
For example, when the porous base used in the present invention comprises a composite of a porous film of polypropylene resin or polyethylene resin and a paper or a nonwoven fabric excellent in heat resistance, there is obtained a separator for non-aqueous electrolyte batteries which is high in safety with having both the shutdown function of the porous film and the heat resistance of the paper or non-woven fabric.
Furthermore, when a porous film comprising polypropylene resin or polyethylene resin is bonded to a woven fabric, nonwoven fabric or paper partially or at the whole surface to form a composite, there is obtained a separator for non-aqueous electrolyte batteries which has the shutdown function of the porous film and is high in strength, good in rollability together with electrodes and excellent in battery fabricability.
When the porous base used in the present invention comprises a composite of a nonwoven fabric mainly composed of polypropylene resin or polyethylene resin and a paper or a nonwoven fabric excellent in heat resistance, there is also obtained a separator for non-aqueous electrolyte batteries which has both the shutdown function of polypropylene or polyethylene and the heat resistance of the paper or nonwoven fabric and is high in strength, good in rollability with electrodes and excellent in battery fabricability.
Especially, when the porous base used in the present invention contains a non-woven fabric, there is obtained a separator for non-aqueous electrolyte batteries having many functions such as excellent electrolyte retention, high tear strength, penetration strength and heat resistance, and excellent dimensional stability after immersed in the electrolyte. Thus, this is preferred.
Content of organic fibers in the porous base containing organic fibers in the present invention is preferably 5-100% by weight, more preferably 10-100% by weight. If the content of organic fibers is less than 10% by weight, especially less than 5% by weight, thickness of the porous base can hardly be reduced and the separator is low in folding strength and penetration and has problems in adhesion to electrodes and rollability with electrodes.
As the inorganic fibers used in the present invention, mention may be made of alumina fibers, alumina-silica fibers, rock wool, glass fibers, micro-glass fibers, zirconia fibers, potassium titanate fibers, alumina whiskers, aluminum borate whiskers and the like.
As the alumina fibers, alumina-silica fibers and rock wool, preferred are those having a fiber diameter of less than several xcexcm and a fiber length of several tens of xcexcm to several hundreds of xcexcm for making a separator for non-aqueous electrolyte batteries which has a uniform thickness. The fiber diameter is more preferably 3 xcexcm or less.
The alumina fibers are fibers mainly composed of alumina. The methods for producing the alumina fibers include the inorganic salt method which comprises spinning a spinning solution comprising a mixture of an aqueous solution of an aluminum salt and a water-soluble polysiloxane and firing the resulting fibers at 1000xc2x0 C. or higher in the air, the sol method which comprises spinning an alumina sol or silica sol and firing the resulting fibers, the prepolymer method which comprises dry-spinning a mixture of a solution containing polyaluminoxane and a silicic acid ester and firing the resulting precursor fibers at 1000xc2x0 C. or higher in the air, and the slurry method which comprises dry-spinning a slurry containing xcex1-Al2O3 fine powders of 0.5 xcexcm or less, firing the resulting precursor fibers at 1000xc2x0 C. or higher and then passing the fibers through a gas flame of 1500xc2x0 C. to sinter the crystal grains.
The alumina fibers are commercially available, for example, in the name of Saffil from ICI (England) and Denka alsen from Denki Kagaku Kogyo K.K.
The alumina-silica fibers are fibers of 40-60% in alumina content and 60-40% in silica content, and are produced, for example, by the following methods. An alumina-silica raw material comprising kaolin calcination product, bauxite alumina, siliceous sand, silica powder and the like to which, if necessary, borate glass, zirconia, chromium oxide and the like are added is molten at a high temperature and made into fibers by blowing method which comprises blowing compressed air or steam jet against the melt or spinning method which utilizes centrifugal force of a rotor rotating at a high speed.
The rock wool is produced, for example, by the following method. A blast furnace slag as a main raw material and silica, dolomite, limestone and the like are molten by heating at 1500-1600xc2x0 C. in an electric furnace and the resulting homogeneous melt is dropped on a high-speed rotator at 1400xc2x0 C. to make it into fibers.
The micro-glass fibers are superfine glass fibers produced by steam spraying method, spinning method, flame introduction method, rotary method and the like, and generally have an average fiber diameter of 5 xcexcm or less.
As the micro-glass fibers, mention may be made of those which comprise borosilicate glass containing, as constitutive components, silicon dioxide (SiO2), sodium oxide (Na2O), calcium oxide (CaO), aluminum oxide (Al2O3), magnesium oxide (MgO), potassium oxide (K2O) and the like, E glass containing substantially no sodium oxide, and silica glass comprising high purity silicon dioxide. However, sodium is substituted for lithium which is an active material to cause decrease of capacity during a long term storage of non-aqueous electrolyte batteries. Therefore, preferred are those which comprise E glass or silica glass.
As micro-glass fibers comprising E-glass and containing no sodium oxide, mention may be made of, for example, E-FIBER commercially available from Schuller Co., Ltd. (U.S.A.).
As the micro-glass fibers comprising silica glass, especially preferred are those which comprise silica glass containing at least 99% by weight of silicon dioxide (in terms of SiO2). Such micro-glass fibers are commercially available, for example, in the name of Q-FIBER from Schuller Co., Ltd. (U.S.A.).
Average fiber diameter of the micro-glass fibers is preferably 3 xcexcm or less. Nonwoven fabrics of high uniformity free from holes of several tens of xcexcm to several hundreds of xcexcm in diameter called pinholes can be produced by using the micro-glass fibers of the smaller average fiber diameter.
In case glass fibers having a large average fiber diameter are used in place of the micro-glass fibers, a porous base non-uniform in thickness and having many pinholes is obtained, and if this is used as a separator for non-aqueous electrolyte batteries, the electrical resistance in width direction of the separator also becomes non-uniform to cause short-circuit between electrodes due to the pinholes.
Content of the inorganic fibers in the porous base in the present invention is not limited, but is preferably 95% by weight or less, especially preferably 50% by weight or less. If it is more than 50% by weight, especially, more than 95% by weight, folding strength or interlaminar strength of the porous base is low to cause a problem in rollability with electrodes.
When the porous base in the present invention contains these inorganic fibers, dimensional stability at high temperatures increases and, hence, there is obtained a separator for non-aqueous electrolyte batteries which is excellent in heat resistance.
The heat resistant organic fibers used in the present invention mean those fibers which do not melt and decompose even at 250xc2x0 C. and are not deteriorated so much even after stored for longer than 1 month in a high temperature atmosphere of 200xc2x0 C.
Specifically, mention may be made of polyarylate fibers, polyphenylene sulfide fibers, polyether ketone fibers, polyimide fibers, polyether sulfone fibers, polyamide fibers, aramid fibers, polyamideimide fibers, polyether imide fibers and the like.
When the porous base used in the present invention contains heat resistant organic fibers having a melting point or a heat decomposition temperature of 250xc2x0 C. or higher, dimensional stability at high temperature increases as compared with when it does not contain such organic fibers, and a separator for non-aqueous electrolyte batteries excellent in heat resistance is obtained.
As the organic fibers at least a part of which are fibrillated to a fiber diameter of 1 xcexcm or less, there is no limitation as far as they can be fibrillated, but preferred are vegetable fibers, vegetable fiber pulp, bacteria cellulose produced by micro-organisms, rayon, polyolefin fibers, polyamide fibers, aramid fibers, and polyarylate fibers.
As the vegetable fibers and vegetable fiber pulp, mention may be made of wood pulp and non-wood fibers or non-wood pulp such as straw, bagasse, paper mulberry, mitsumata (Edgeworthia papyrifera), Manila hemp, esparto, cotton linter, ganpi (Wikstroemia sikokiana Fr. Et Sav.), jute, bamboo, ditch reed, papyrus, kenaf and ramie.
The bacteria cellulose produced by micro-organisms in the present invention is that which contains cellulose and heteropolysaccharides having cellulose as main chain and glucans such as xcex2-1,3 and xcex2-1,2. Constitutive components other than cellulose in the case of heteropolysaccharides are hexoses, pentoses and organic acids such as mannose, fructose, galactose, xylose, arabinose, rhamnose and glucuronic acid. These polysaccharides comprise a single substance in some case, and two or more polysaccharides bonded through hydrogen bond in some case. Either of them may be utilized.
Any bacteria celluloses produced by micro-organisms mentioned above may be used in the present invention.
The micro-organisms which produce the bacteria cellulose are not limited, and there may be used Acetobacter aceti subsp. xylinium ATCC 10821, A. pasteurian, A. rancens, Sarcina ventriculi, Bacterium xyloides, Pseudomonas bacteria, Agrobacterium bacteria, and the like that produce the bacteria cellulose.
Production and accumulation of bacteria cellulose by cultivation of these micro-organisms can be carried out in accordance with the general methods for culture of bacteria. That is, micro-organisms are inoculated in usual enriched media containing a carbon source, a nitrogen source, an inorganic salt and, if necessary, organic minor nutrients such as amino acid and vitamins, and then subjected to stationary culture or gentle spinner culture.
Then, the thus produced and accumulated bacteria cellulose is macerated to prepare an aqueous slurry. The maceration can be easily performed by a rotary macerator or mixer. The resulting bacteria cellulose macerated product is markedly higher in bonding ability between fibers than other cellulose fibers. Therefore, addition of the bacteria cellulose in only a small amount to other organic or inorganic fibers can give a porous base of high strength.
When the porous base used in the present invention contains the bacteria cellulose produced by micro-organisms, there is obtained a separator for non-aqueous electrolyte batteries which is high in mechanical strength and gives excellent battery fabricability at the time of fabrication of batteries.
As for the organic fibers at least a part of which are fibrillated to a fiber diameter of 1 xcexcm or less used in the present invention, commercially available fibrillated fibers which are already fibrillated may only be macerated by a pulper or the like, and, furthermore, unfibrillated fibers may be previously fibrillated by a high-pressure homogenizing apparatus. In this case, also at least a part of the fibers may only be fibrillated to a fiber diameter of 1 xcexcm or less.
In the case of carrying out the fibrillation using a high-pressure homogenizing apparatus, it can be performed, for example, in the following manner.
Organic fibers cut to a fiber length of 5 mm or less, preferably 3 mm or less are dispersed in water to prepare a suspension. Concentration of the suspension is 25% by weight at maximum, preferably 1-10% by weight, more preferably 1-2% by weight. This suspension is introduced into a high-pressure homogenizing apparatus used for preparation of emulsion or dispersion and repeatedly passed through the homogenizing apparatus under application of a pressure of at least 100 kg/cm2, preferably 200-500 kg/cm2, more preferably 400-500 kg/cm2, during which a shearing force generated by rapid collision against the wall of apparatus and rapid speed reduction is applied to the organic fibers, and the effect is applied mainly as a force to cause tearing and loosening in the direction parallel to the fiber axis, resulting in gradual fibrillation.
Content of the organic fibers at least a part of which are fibrillated to a fiber diameter of 1 xcexcm or less in the porous base used in the present invention has no limitation in the case of the fibers being natural fibers or bacteria cellulose produced by micro-organisms, but is preferably less than 50% by weight in the case of the fibers comprising synthetic polymers.
When the organic fibers at least a part of which are fibrillated to a fiber diameter of 1 xcexcm or less in the present invention are natural fibers or bacteria cellulose produced by micro-organisms, a porous base excellent in strength can be obtained even when the organic fibers comprise 100% by weight of natural fibers or bacteria cellulose because self-binding force of these fibers due to hydrogen bond is strong.
On the other hand, when the organic fibers at least a part of which are fibrillated to a fiber diameter of 1 xcexcm or less in the present invention comprise synthetic polymers, binding force of the fibers per se is weak, and if the porous base is produced using 50% by weight or more, especially 100% by weight of these fibers, the fibers fall off from the porous base or the porous base is very low in tear strength or penetration strength and has a problem in rollability with electrodes.
When at least a part of the organic fibers contained in the porous base used in the present invention are fibrillated to a fiber diameter of 1 xcexcm or less, no pinholes are produced and mechanical strength is improved, and, therefore, a separator for non-aqueous electrolyte batteries which gives excellent battery fabricability such as rollability with electrodes at fabrication of batteries can be produced.
The polyvinyl alcohol in the present invention includes powdery or fibrous polyvinyl alcohol, either of which can be used. The powdery polyvinyl alcohol is made into an aqueous solution, which is allowed to contact with a porous base by impregnation, coating, spraying or the like and dried, whereby a film can be formed on the surface of the porous base.
The fibrous polyvinyl alcohol is mixed with other fibers and the mixture is made into an aqueous slurry. This is made into a sheet by wet paper making process or the like, and then the sheet is dried to form a film. For example, vinylon fibers manufactured by Kuraray Co., Ltd. can be exemplified.
Content of the polyvinyl alcohol in the porous base in the present invention is preferably 50% by weight or less, especially preferably 30% by weight or less.
If the content of the polyvinyl alcohol is more than 30% by weight, particularly more than 50% by weight, area of the film formed on the surface of the porous base becomes too large, and pores necessary for ion permeation are apt to be unevenly distributed, and, in some case, the porous base becomes filmy and the pores are ruptured.
The polyvinyl alcohol acts as a binder and strongly binds with organic fibers or inorganic fibers constituting the porous base. Thus, not only a separator for non-aqueous electrolyte batteries which is excellent in mechanical strengths such as tensile strength, tear strength and penetration strength can be obtained, but also a film is formed by heat on the surface of the separator and the pore diameter can be made small, and as a result, a thinner separator for non-aqueous electrolyte batteries can be obtained.
The heat-fusible fibers in the present invention include heat-meltable type of the fibers per se being partially or wholly molten by heat to produce binding force between fibers, a type of the fibers per se being partially or wholly dissolved in water or hot water and producing bonding force between fibers in the drying stage, and other types. These are used each alone or in admixture of two or more depending on the purpose.
As specific examples of these fibers, mention may be made of polyvinyl alcohol fibers, polyester fibers, polypropylene fibers, polyethylene fibers, composite fibers comprising polyethylene and polypropylene, composite fibers comprising polypropylene and ethylene-vinyl alcohol copolymer, and the like.
As the organometallic compounds used in the present invention, mention may be made of organosilicon compounds, organotitanium compounds, organoaluminum compounds, organozirconium compounds, organozirco-aluminate compounds, organotin compounds, organocalcium compounds, organonickel compounds, and the like. Among them, organosilicon compounds, organotitanium compounds, organoaluminum compounds, organozirconium compounds, and organozirco-aluminate compounds are preferred because these compounds are excellent in the effect of blocking polar groups which adversely affect battery characteristics.
Among them, especially preferred are organosilicon compounds because they can be easily handled in aqueous system, are excellent in film-forming ability when dried or cured by heating and great in the effect of blocking polar groups.
As the organosilicon compounds used in the present invention, preferred are organosilanes or organopolysiloxanes having at least one hydrolyzable group or functional group selected from chlorine group, fluorine group, acetoxy group, alkoxy group, vinyl group, amino group, epoxy group, mercapto group and methacryl group.
Examples of these organosilanes are organohalosilanes such as trimethylchlorosilane, methyltrichlorosilane, ethyltrichlorosilane, propyltrichlorosilane, hexyltrichlorosilane, dodecyltrichlorosilane, dimethyldichlorosilane, methyldichlorosilane, dimethylchlorosilane, dimethylvinylchlorosilane, methylvinyldichlorosilane, methylchlorodisilane, octadecyltrichlorosilane, trimethylchlorosilane, t-butylmethylchlorosilane, dichloroethylphenylsilane, triphenylchlorosilane, methyldiphenylchlorosilane, diphenyldichlorosilane, methylphenyldichlorosilane, phenyltrichlorosilane, chloromethyldimethylchlorosilane, chlorodifluoromethylsilane, dichlorodifluoromethylsilane, and dichlorodifluoropropylsilane; organoacetoxysilanes such as acetoxytrimethylsilane, diacetoxydimethylsilane, acetoxytripropylsilane, ethyltriacetoxysilane and methyltriacetoxysilane; organoalkoxysilanes such as methoxytrimethylsilane, dimethyldimethoxysilane, trimethylphenoxysilane, methyltrimethoxysilane, methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, vinyltrimethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, diphenyldimethoxysilane, diphenyldiethoxysilane, methylvinyldimethoxysilane, methylvinyldiethoxysilane, dimethylvinylmethoxysilane, dimethylvinylethoxysilane, tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, butyltriethoxysilane, pentyltriethoxysilane, hexyltriethoxysilane, heptyltriethoxysilane, octyltriethoxysilane, nonyltriethoxysilane, decyltriethoxysilane, undecyltriethoxysilane, dodecyltriethoxysilane, tridecyltriethoxysilane, tetradecyltriethoxysilane, pentadecyltriethoxysilane, hexadecyltriethoxysilane, heptadecyltriethoxysilane, octadecyltriethoxysilane, butyltrimethoxysilane, pentyltrimethoxysilane, hexyltrimethoxysilane, nonyltrimethoxysilane, decyltrimethoxysilane, undecyltrimethoxysilane, dodecyltrimethoxysilane, tridecyltrimethoxysilane, tetradecyltrimethoxysilane, pentadecyltrimethoxysilane, hexadecyltrimethoxysilane, heptadecyltrimethoxysilane and octadecyltrimethoxysilane; organosilazanes such as hexamethyldisilazane, 1,3-diphenyltetramethyldisilazane, octamethylcyclotetrasilazane, 1,1,3,3-tetramethyldisilazane and hexamethylcyclotrisilazane; isocyanate silanes such as trimethylsilyl isocyanate, dimethylsilyl isocyanate, methylsilyl triisocyanate, vinylsilyl triisocyanate, phenylsilyl triisocyanate, tetraisocyanate silane and ethoxysilane triisocyanate; silane coupling agents such as vinyltriethoxysilane, vinyltriacetoxysilane, vinyltris(xcex2-methoxyethoxy)silane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N-(xcex2-aminomethyl)-xcex3-aminopropylmethyldiethoxysilane, N-(xcex2-aminoethyl)-xcex3-aminopropyltrimethoxysilane, xcex3-methacryloxypropyltrimethoxysilane, xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltriethoxysilane, xcex3-chloropropyltrimethoxysilane, xcex3-glycidoxypropyltrimethoxysilane, xcex3-phenylaminopropyltrimethoxysilane and N-phenyl-xcex3-aminopropyltrimethoxysilane. The organosilanes are not limited to those exemplified above.
Examples of the organopolysiloxanes are methylpolysiloxane, dimethylpolysiloxane, methylpolycyclosiloxane, methylphenylpolysiloxane, methylhydrogenpolysiloxane, methylstyrene-modified silicones, long chain alkyl-modified silicones, polyether-modified silicones, amino-modified silicones, carbinol-modified silicones, epoxy-modified silicones, carboxyl-modified silicones, mercapto-modified silicones and methacryl-modified silicones. The organopolysiloxanes are not limited to those exemplified above.
The organosilicon compounds in the present invention are dissolved in solvents such as water, ethanol, methanol, chloroform or the like and hydrolyzed or condensed, and the resulting products are used.
The organosilicon compounds can react with polar groups such as hydroxyl group and silanol group by subjecting them to hydrolysis. Moreover, condensation between organosilicon compounds also proceeds to become an oligomer, and, hence, a film is formed and can be allowed to adhere.
In the present invention, alkalis or acids may be used as catalysts for the acceleration of hydrolysis or condensation of the organosilicon compounds.
As the organotitanium compounds used in the present invention, mention may be made of titanium alkoxides such as tetramethoxytitanium, diisopropoxybis(ethylacetoacetate)titanium, tetraisopropoxytitanium, tetra-n-butoxytitanium, butyl titanate dimer, tetrakis(2-ethylhexyloxy)titanium, tetramethyl titanate and tetrastearyloxytitanium; titanium acylates such as polyhydroxytitanium stearate and polyisopropoxytitanium stearate; titanium chelates such as diisopropoxybis(acetylacetonato)titanium, titanium lactate, isopropoxy(2-ethyl-1,3-hexanediolato)titanium, di(2-ethylhexoxy)bis(2-ethyl-1,3-hexanediolato)titanium, di-n-butoxybis-(triethanolaminato)titanium and tetraacetylacetonatetitanium; and titanium polymers such as tri-n-butoxytitanium monostearate polymer, tetra-n-butoxytitanium polymer and titanium phosphate polymer. The organotitanium compounds are not limited to these examples.
The organotitanium compounds in the present invention are used as hydrolyzates. Specifically, they mean organotitanium compounds dissolved in water or organic solvents. The organic solvents include ethanol, isopropanol, hexane, toluene, benzene, and the like.
For example, hydrolysis of titanium alkoxides results in precipitation of titanium oxide. In titanium acylates, unhydrolyzed acyl groups remain and form a long arrangement to cover the surface. Lower acylates are condensed to become polymers. Titanium chelates are more difficult to hydrolyze than titanium alkoxides, but show the similar reaction to titanium alkoxides.
Therefore, when a hydrolyzate of these organotitanium compounds is applied to a separator for non-aqueous electrolyte batteries, there is formed a film of titanium oxide or a film of the organotitanium compound having organic group.
As the organoaluminum compounds used in the present invention, mention may be made of aluminum isopropylate, aluminium tri-secondary-butoxide, aluminum ethylate, aluminum ethylacetoacetatediisopropylate, aluminum tris(ethylacetoacetate), aluminum tris(acetylacetonate), aluminum bisethylacetoacetatemonoacetylacetonate, acetoalkoxyaluminum diisopropylate, and the like. The organoaluminum compounds are not limited to these examples.
As the organozirconium compounds used in the present invention, mention may be made of zirconium n-propoxide, zirconium n-butoxide, zirconyl acetate, zirconium acetylacetonate, zirconium butoxyacetylacetonate, zirconium bisacetylacetonate, zirconium ethylacetoacetate, zirconium acetylacetonatobisethylacetoacetate, and the like. The organozirconium compounds are not limited to these examples.
The organozirco-aluminate compounds include zirco-aluminate coupling agents.
Since the polar groups which adversely affect the battery characteristics can be blocked by applying these organometallic compounds, not only high energy density and excellent cycle life can be obtained, but also since the organometallic compounds are applied in the form of a film, heat resistance of the separator for non-aqueous electrolyte batteries is improved.
For example, when a separator for non-aqueous electrolyte batteries to which an organosilicon compound is uniformly applied is left to stand at a high temperature of 500xc2x0 C. for a long period of time, only the organic matter bonding to silicon is oxidized and deteriorated and inorganic silica remains. Therefore, even when the separator for non-aqueous electrolyte batteries is kept in the state of such high temperature, the silica functions to maintain the shape of the separator, and especially the shrink in Z direction is inhibited by the silica remaining in the separator for non-aqueous electrolyte batteries. Thus, a separator for non-aqueous electrolyte batteries which is excellent in heat resistance and can prevent internal short-circuit of electrodes can be obtained. Furthermore, even when the organosilicon compound is not uniformly applied to the whole of the separator for non-aqueous electrolyte batteries, if the organosilicon compound is partially applied to the separator, for example, in the case the separator containing a material to which the organosilicon compound is applied, heat resistance of the separator is improved than when no organometallic compound is applied. The same effect as in the case of the organometallic compounds can be obtained by the organotitanium compounds, organoaluminum compounds and organozirconium compounds.
These organometallic compounds may be used each alone or in admixture of two or more. Moreover, after one or a mixture of two or more of them are applied to a porous base, another organometallic compound may be further applied thereto.
The separator for non-aqueous electrolyte batteries in the present invention is produced in the following manner.
That is, a porous base comprising at least one material selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper is allowed to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying and curing with heating to apply the organometallic compound to the porous base.
When the porous base in the present invention is a composite, at least one porous base (A) selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper or (A) and a porous base (B) containing no organic fibers are allowed to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying and curing with heating to previously apply the organometallic compound to the composite, and then a composite (C) comprising the combination of (A) or the combination of (A) and (B) is formed.
Another method is that after the composite (C) is formed which comprises the combination of at least one porous base (A) selected from a porous film, a woven fabric and nonwoven fabric containing organic fibers and a paper or comprises the combination of (A) and a porous base (B) containing no organic fibers, the composite (C) is allowed to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying and curing with heating to apply the organometallic compound to the composite (C).
Methods for producing the composite (C) in the present invention include a method which comprises combining a porous film, a woven fabric, a nonwoven fabric and a paper depending on the purpose, laminating them without bonding the layers and rolling the laminate together with electrodes at the time of fabrication of batteries and the said method where said laminate is formed with bonding the layers partially or at the whole surface to make a multi-layer body.
As the method for bonding the porous film, the woven or nonwoven fabric containing organic fibers and the paper in combination, effective are a method of hot pressing them using a hot calender, a hot soft calender, a hot embossing calender or the like, and a method of hot pressing them using a hot melt adhesive, split fabric, sizing fabric, polyethylene fine particles, and the like.
Similarly, for bonding at least one material selected from the porous film, the woven or nonwoven fabric containing organic fibers and the paper and the porous base containing no organic fibers in combination, effective are a method of hot pressing them using a hot calender, a hot soft calender, a hot embossing calender or the like, and a method of hot pressing them using a hot melt adhesive, split fabric, sizing fabric, polyethylene fine particles.
The temperature at which the porous base is produced by hot pressing in the present invention varies depending on the kinds of resins or organic fibers contained in the porous base and the hot pressing treatment is carried out at a temperature higher than Tg and lower than the melting point of the resin or organic fibers. The hot pressing treatment using hot calender or hot soft calender is preferred because the layers are bonded through the whole surface thereof and hence no interlaminar separation occurs at rolling together with electrodes. In the case of the hot pressing treatment using a hot embossing calender, the layers are partially bonded at the embossed pattern. Therefore, the composite can be formed without causing much decrease of gas permeability of the porous base, and if a sufficient bonding area is obtained, a strength causing no interlaminer separation at the rolling together with electrodes can be obtained. This is preferred.
Furthermore, when the porous base is produced by hot pressing treatment using a hot melt adhesive in the form of a sheet or a split fabric or a sizing fabric comprising polyolefin resin, polyester resin or polyamide resin or polyethylene fine particles, thereby to partially bond the layers, the composite can be obtained without causing much decrease of gas permeability of the porous base. Thus, this is preferred.
Other than these methods, as a method of bonding nonwoven fabrics per se, papers per se, and nonwoven fabric and paper, effective is a method of making them to a multi-layer by wet paper making process with each other, followed by drying or curing with heating to bond them or a method of water jet entangling by jetting a high-pressure columnar water.
When the separator for non-aqueous electrolyte batteries according to the present invention comprises a porous base of nonwoven fabric or a paper to which the organometallic compound is applied, the organometallic compound may be applied to the porous base by allowing a commercially available nonwoven fabric or paper or a ready-made nonwoven fabric or paper to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying or curing by heating. However, the polar groups which adversely affect the battery characteristics can be efficiently blocked by producing the separator in the following manner.
That is, a wet sheet obtained by wet paper making process or this sheet after dried is allowed to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying or curing by heating to apply the organometallic compound to the sheet.
By employing the wet paper making process, there is obtained a separator for non-aqueous electrolyte batteries which is superior to the conventional separators in tear strength and penetration strength.
The paper making machines used for the wet paper making include Fourdrinier paper machines, cylinder paper machines, inclined paper machines, combination paper machines comprising a combination of two or more, and the like.
According to the present invention, a fiber slurry containing the organometallic compound is beaten or macerated, and then the fiber slurry alone or a mixture of the fiber slurry with other fiber slurry is subjected to wet paper making process, followed by drying or curing with heating to apply the organometallic compound to the sheet, whereby the organometallic compound is also uniformly applied to binding points of the fibers, thereby increasing the effect of blocking the polar groups which adversely affect the battery characteristics.
Furthermore, according to the present invention, a fiber slurry containing the organometallic compound is beaten or macerated, then the fiber slurry alone or a mixture of the fiber slurry with other fiber slurry is subjected to wet paper making process, and then the resulting wet sheet or this sheet after dried is allowed to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying or curing with heating to apply the organometallic compound to the sheet. As a result, the effect of blocking the polar groups which adversely affect the battery characteristics is further increased and a separator for non-aqueous electrolyte batteries which is high in energy density and excellent in cycle life can be obtained.
Especially, when organic fibers at least a part of which are fibrillated to a fiber diameter of 1 xcexcm or less and which contain the polar groups adversely affecting the battery characteristics are contained as a material constituting the separator for non-aqueous electrolyte batteries, a fiber slurry containing the fibers is mixed with the organometallic compound and the mixture is beaten or macerated thereby to apply the organometallic compound uniformly to the whole fibers. Thus, the effect to block the polar groups which adversely affect the battery characteristics is great.
As far as the effect to block the polar groups is not greatly lowered, for the purpose of increasing strength of the porous base, the fiber slurry in the present invention may be mixed with various adhesives, for example, thermoplastic resins such as of vinyl acetate type, vinyl chloride type, polyvinyl alcohol type, polyvinyl acetal type, acryl type, polyamide type, and ethylene-vinyl acetate copolymer; thermosetting resins such as of urea type, melamine type, phenolic type, epoxy type, polyurethane type, polyester type, polyaromatic type, and resorcinol type; and elastomers such as of chloroprene type, nitrile rubber type, butyl rubber type, polypropylene type and silicone rubber type.
In stead of mixing with the fiber slurry, the adhesives may be applied by allowing the wet sheet obtained by wet paper making process or this sheet after dried to contact with the adhesives by impregnation, coating or spraying, followed by drying or curing with heating.
The methods for beating or macerating the fiber slurry containing the organometallic compound in the present invention include a method of beating by a beating machine such as beater or refiner and a method of macerating by pulper or the like.
Concentration of the organometallic compound in the solution is usually 0.1-5%. If the concentration is lower than 0.1%, application amount of the organometallic compound is insufficient and polar groups which adversely affect the battery characteristics can be blocked with difficulty, and if it is higher than 5%, the effect of blocking the polar groups no longer changes.
Impregnation of the solution of the organometallic compound is carried out using an impregnating machine, and there are prewetting method, floating method and doctor bar method.
Methods for coating the solution of the organometallic compound in the present invention include those which use coaters such as size press, air doctor coater, blade coater, transfer roll coater, rod coater, reverse roll coater, gravure coater, die coater, and notch bar coater.
Methods for spraying the solution of the organometallic compound in the present invention include those which use spraying apparatuses such as spray.
Application amount of the organometallic compound to the porous base has no particular limitation, but is preferably 0.05% by weight or more, more preferably 0.1% by weight or more based on the weight of the material having the polar group which reacts with the organometallic compound. If the application amount is less than 0.05% by weight, the application amount of the organometallic compound is insufficient and the polar groups which adversely affect the battery characteristics cannot be completely blocked, and as a result the energy density and cycle life tend to be deteriorated. On the other hand, if it is 0.05% by weight or more, the polar groups which adversely affect the battery characteristics can be completely blocked, and as a result the energy density and cycle life of non-aqueous electrolyte batteries are improved. When the application amount is 0.1% by weight or more, the effect does not change with increase of the application amount, but heat resistance of the separator for non-aqueous electrolyte batteries is improved with increase of the application amount and hence there is no special upper limit. However, application of the organometallic compound in a large amount sometimes causes clogging of the separator, resulting in reduction of ion permeability and causes problems in energy density and cycle life. Therefore, the upper limit is preferably 20% by weight, more preferably 10% by weight.
The solution of the organometallic compound is not only applied to the surface of the porous base, but also penetrates into the porous base and can block the polar groups such as hydroxyl group and silanol group which adversely affect the battery characteristics. Therefore, there is obtained a separator for non-aqueous electrolyte batteries which provides high energy density and excellent cycle life.
When the porous base in the present invention contains a nonwoven fabric or a paper, since there are complicated interlockings of the fibers also in the Z direction, the organometallic compound can be widely applied up to the inside of the porous base, resulting in a great effect to inhibit shrinking in the Z direction at high temperatures. Thus, there can be obtained a separator for non-aqueous electrolyte batteries which is excellent especially in heat resistance.
When the maximum pore diameter of the nonwoven fabric or paper used in the present invention is 20 xcexcm or less as measured by bubble point method specified in ASTM F-316-80, the separator is excellent in electrolyte retention, and since charging and discharging can be stably repeated, an excellent cycle life and battery shelf stability can be obtained.
When the porous base in the present invention contain especially the nonwoven fabric, the porous base swells a little after immersed in the electrolyte and is excellent in dimensional stability, and hence area of the electrodes to be incorporated in the battery can be gained and non-aqueous electrolyte batteries of high capacity can be obtained.
Basis weight of the separator for non-aqueous electrolyte batteries of the present invention has no special limitation, but is preferably 5-100 g/m2, more preferably 10-50 g/m2.
Thickness of the separator for non-aqueous electrolyte batteries according to the present invention depends on the thickness of the porous base used and has no special limitation, but is preferably thinner from the point of miniaturization of the batteries. Specifically, the thickness is preferably 10-100 xcexcm, more preferably 20-60 xcexcm for providing such a strength as being not broken at the fabrication of batteries, producing no pinholes and giving a high uniformity. If the thickness is less than 10 xcexcm, percentage of rejects due to short-circuit at the time of fabrication of batteries tends to increase. If it is more than 100 xcexcm, electrical resistance due to the thickness increases, and battery characteristics tends to deteriorate or the energy density tends to considerably decrease.
When the thickness of the separator for non-aqueous electrolyte batteries is greater than desired or void content of the separator is high, it is necessary to reduce the thickness or decrease the void content by secondary processing. This secondary processing include pressing or hot pressing by calenders such as super calender, machine calender, soft calender, hot calender and hot soft calender.
When the separator is subjected to pressing, reversion of the thickness sometimes occurs with time. On the other hand when the separator is subjected to hot pressing, the reversion of the thickness hardly occurs though it depends on the processing temperature, and the separator can easily be adjusted to the desired thickness or void content.
The processing temperature for hot pressing the separator using a hot calender or hot soft calender varies depending on the kind of the resin or organic fibers contained in the porous base, and the separator is processed at a temperature higher than Tg and lower than the melting point of the resin or organic fibers. Especially when heat-fusible fibers are contained, it is necessary to raise the processing temperature to a temperature at which the adhesive force of the heat-fusible fibers is developed. In view of the construction of the organic fibers and the processing conditions, the processing temperature is preferably 50-200xc2x0 C. If the pressing is carried out at lower than 50xc2x0 C., there are sometimes caused such troubles that no sufficient adhesive force is developed, reversion of the thickness occurs with lapse of time, the thickness cannot be reduced to the desired thickness, and cracks are produced. If the pressing is carried out at higher than 200xc2x0 C., the resin or organic fibers per se are sometimes deteriorated with heat to cause decrease of strength or distortion of the separator. Even if the deterioration does not occur, the density of the separator for non-aqueous electrolyte batteries increase too much and sufficient void content cannot be obtained to damage the battery performance.
The timing of subjecting the porous base to the pressing or hot pressing may be either before or after the application of the organometallic compound to the porous base.
In case the porous base used in the present invention is a composite, the timing of subjecting it to the pressing or hot pressing may be either before or after the formation of the composite, but in case porous bases differing greatly in heat resistance or heat shrinkability are made to a composite, the processing is preferably carried out before the formation of the composite because creases or distortion sometimes result if it is carried out after the formation of the composite.
Especially, when the porous base in the present invention is a composite containing a nonwoven fabric, it is preferred to make the composite after the nonwoven fabric is subjected to pressing or hot pressing to adjust the thickness. This is because the surface smoothness of the nonwoven fabric is improved by the pressing or hot pressing to increase the adhesion to the porous base, providing a uniform composite. Especially the hot pressing has a great effect since the surface smoothness and strength of the nonwoven fabric are markedly improved.
For the same reasons, when the porous base in the present invention is a composite containing a paper, it is preferred to make the composite after the paper is subjected to pressing to adjust the thickness and increase the surface smoothness.
When the porous base used in the present invention is a composite of a porous base having the polar group adversely affecting the battery characteristics and a porous base having no such group, the composite may be made after the organometallic compound is applied to only the former porous base.
The timing of subjecting to pressing or hot pressing a porous base comprising a nonwoven fabric or paper produced by allowing a dried sheet obtained by subjecting to wet paper making process a fiber slurry containing or not containing the organometallic compound to contact with a solution of the organometallic compound by impregnation, coating or spraying, followed by drying or curing by heating to apply the organometallic compound to the sheet may be either before or after the application of the organometallic compound.
By the pressing or hot pressing, the separator for non-aqueous electrolyte batteries according to the present invention is not only adjusted to the desired thickness or void, but also is improved in the surface smoothness, and therefore is improved in adhesion to electrodes and gap or space are hardly produced between the electrode and the separator at the time of rolling together with electrodes.
Moreover, pore diameter of the separator can be reduced by the pressing or hot pressing.
Particularly, by the hot pressing, the heat-fusible fibers or the organic fibers or resin of low melting point contained in the separator produce a film, which strongly binds with other fibers or resin to markedly improve tear strength or penetration strength of the separator. Thus, a separator for non-aqueous electrolyte batteries which is very good in rollability with electrodes is obtained, and the pore diameter can further be reduced and the maximum pore diameter can be reduced to 20 xcexcm or less, further 10 xcexcm or less as measured by the bubble point method specified in ASTM F-316-80.
Furthermore, by carrying out the hot pressing, the heat-fusible fibers or the organic fibers or resin of low melting point contained in the separator produce a film, which causes strong binding with other fibers or resin, and therefore the separator swells a little after immersed in the electrolyte and is excellent in dimensional stability, and area of electrode to be incorporated into the battery can be gained, and as a result, non-aqueous electrolyte batteries of high capacity can be obtained.